1. Field of the Invention
The present invention relates to a demodulating apparatus for demodulating a modulated signal obtained by modulating a plurality of carriers having different frequencies by using data.
2. Description of the Related Art
As a modulating apparatus for demodulating a modulated signal obtained by modulating a plurality of carriers having different frequencies by using data, an apparatus for demodulating an orthogonal frequency division multiplex (OFDM) modulated signal (hereinafter referred to as an OFDM modulated signal) employed in a digital audio broadcasting (DAB) or the like carried out in Europe has been proposed.
In this OFDM modulation, data such as audio data or the like are encoded by using a modulated signal using a large number of carriers whose frequency components have orthogonal relationships with one another, and the encoded data are allocated to the respective carriers to thereby modulate the respective carriers. Further, a digital signal in a frequency domain formed of the respective modulated carriers is converted by inverse fast Fourier transform into a digital signal in a time domain, and then the digital signal in the time domain is converted into an analog signal. When the modulated signal is demodulated, the OFDM modulated signal is converted into a digital signal, and then the digital signal is subjected to fast Fourier transform, thus the encoded data allocated to the respective carriers being obtained.
In the OFDM modulation in the DAB, the respective carriers are subjected to a quadrature phase shift keying (QPSK) modulation by allocating one carrier to two-bit data each. Therefore, this modulation is referred to as OFDM-QPSK modulation.
In the OFDM modulation, the point number of the fast Fourier transform corresponds to the number of carriers. According to a DAB standard, the-point number is changed depending upon modes. In modes 1, 2, 3, 4 thereof, the point number is 1536, 384, 192, 768, respectively. Accordingly, if the mode is the mode 1, it is possible to transmit data of 2 (bits).times.1536=3072 (bits) by the OFDM modulation. This transmission unit is called as a symbol. In the modes 1, 2 and 4, a group of seventy-six symbols is referred to as a frame, and in the mode 3, a group of one hundred and fifty-three symbols is referred to as a frame. Each of the above numbers of the symbols in one frame does not include the number of a null symbol.
Synchronization is usually adjusted by adding several synchronization symbols each formed of synchronization adjustment data to a head of one frame. According to the DAB standard, of the seventy-seven symbols or one hundred and fifty-four symbols (both of which include the null symbol and are respectively those in the modes 1, 2 and 4 and the mode 3), two symbols including a null symbol are employed as the synchronization symbol. A demodulation side (reception side) compares amplitudes of a real-number portion and an imaginary-number portion of the synchronization data subjected to the fast Fourier transform with a previously held amplitude of the normal synchronization data (i.e., an amplitude of synchronization data practically set by a transmission side) to calculate a difference between synchronization phases upon fast Fourier transform. Further, the demodulation side adjusts a timing of the fast Fourier transform in response to the synchronization phase difference to obtain synchronization. In this method, since the synchronization can be obtained only once in one frame, it disadvantageously takes a considerable time to obtain synchronization.
A synchronization generating processing for generating a synchronization signal on the demodulation side (reception side) based on the signal obtained by converting the OFDM modulated signal into an analog signal will be described. The above symbol will be described with reference to FIG. 1. The symbol is formed of a guard interval positioned on its head side and an effective symbol positioned on its end side. The effective symbol includes a period having correlation to the guard interval, i.e., a period having the same signal portion and the same interval on the end side thereof.
An original signal (e.g., a symbol of mode 1) shown in FIG. 2A is delayed by a time corresponding to an effective symbol period in the symbol. A signal of a period on the end side of the original signal shown in FIG. 2A corresponding to the guard interval (whose period is about 1/5 as long as the symbol period) and a signal of a guard interval of the delayed signal shown in FIG. 2B are stored in a memory, and then both of the signals stored in the memory are multiplied with each other by using a digital signal processor (DSP) or the like. A product of the multiplication is integrated by a low-pass filter, thereby correlation being detected. Then, since the signal on the end side of the effective symbol of the symbol of the original signal which has the same interval and the same signal portion as those of the guard interval and the signal of the guard interval of the symbol of the delayed signal are the same, i.e., have the same correlation with each other, a correlation signal having a rectangular waveform (shown in FIG. 2C) is obtained.
If the correlation signal is subjected to interval integration (moving average) using a width of time corresponding to the guard interval in the symbol, then there can be obtained as shown in FIG. 2D a signal which has an axially symmetric triangular waveform and which starts being inclined upward at the rising edge of the correlation signal indicative of correlation and having a rectangular waveform and starts being inclined downward at the trailing edge of the correlation signal. As shown in FIG. 2D, the signal obtained by subjecting the correlation signal to interval integration is compared with a threshold level TH slightly lower than an amplitude level of the practically obtained triangular wave signal, and thereby a noise is removed therefrom to obtain only the normal signal obtained by subjecting the correlation signal to interval integration. A peak position of the signal obtained by subjecting the correlation signal to interval integration is discriminated, thereby a time synchronization signal synchronized with the discriminated peak position being generated.
A guard interval removal signal is generated based on the time synchronization signal. The guard interval in the signal obtained by converting the OFDM modulated signal into an digital signal is removed by the guard interval removal signal and then the timing of the fast Fourier transform is controlled based on the time synchronization signal. The time synchronization signal is also used when data of a signal subjected to the fast Fourier transform is decoded.
A phase of the time synchronization signal is discriminated. Based on the discrimination result, a frequency synchronization signal is generated. The time synchronization signal is converted into an analog signal, i.e., an auto frequency control (AFC) signal (frequency control signal). An oscillation frequency of a local oscillator for frequency conversion is controlled based on the AFC signal. A RF reception signal is thus frequency-converted into a signal having an intermediate frequency, and then subjected to the above A/D conversion.
At present, the known DAB signals are signals of modes 1, 2, 3, 4. A determined basic period thereof is T (=1/2048000 sec=0.00048828 nsec). FIG. 3 shows a structure of the DAB signal of the mode 1, by way of example. In FIG. 3, the basic period T and time are both indicated. One frame of the mode 1 DAB signal is 196608 T (=96 msec) and formed of one null symbol (symbol number 1=0) having an interval of 2656 T (=1.297 msec) and seventy-six symbols (symbol numbers 1=1 to 76) succeeding the null symbol and each having an interval of 2552 T (=1.246 msec).
Each of symbols having the symbol numbers 1=1 to 76 is formed of a guard interval having an interval of 504 T (=246 .mu.sec) and an effective symbol at-the succeeding position and having an interval of 2048 T (+1 msec). Effective symbols of the respective symbols having the symbol numbers 1=1 to 76 include multicarriers of the number of k=1536 having frequencies different from one another. A carrier indicated by 0 is a carrier having a center frequency (a period of the carrier is T). A carrier indicated by 1536/2 (=766) is a carrier having a maximum frequency, and a carrier indicated by -1536/2 (=-766) is a carrier having a minimum frequency. A data amount of one symbol includes 1536 waves, and a data amount thereof is 1536.times.2 bits, i.e, 48 capacity units (CU).times.64 bits.
The whole symbols having the symbol numbers 1=1 to 76 are referred to as an OFDM symbol.
In case of the mode 1, for example, the null symbol having the symbol number 1=0 and the symbol having the symbol number 1=1 are referred to as time frequency and phase reference (TFPR) symbols, respectively. A set of these two symbols is referred to as a synchronization channel (sync. channel). Symbols having the symbol numbers 1=2 to 4 are referred to as fast information channels (FIC). The whole FICs are divided into twelve fast information blocks (FIB). The remaining symbols having the symbol numbers 1=5 to 76 are classified into four common interleaved frames (CIF).
An interval of each symbol of the DAM signal is different depending upon the mode. An interval of each symbol of the mode 2 is 1/4 as long as the interval of each symbol of the mode 1. An interval of each symbol of the mode 3 is 1/8 as long as the interval of each symbol of the mode 1. An interval of each symbol of the mode 4 is 1/2 as long as the interval of each symbol of the mode 1.
Specifically, in the mode 1, the interval of each of the symbols excluding the null symbol is 2552 T (=1.246 msec) as described. In the mode 2, the interval of each of the symbols excluding the null symbol is 638 T (=2552T/4)(=312 .mu.sec (=1.246/4 msec)). In the mode 3, the interval of each of the symbols excluding the null symbol is 319 T (=2552T/8)(=156 .mu.sec (=1.246/8 msec)). In the mode 4, the interval of each of the symbols excluding the null symbol is 1276 T (=2552T/2)(=623 .mu.sec (=1.246/2 msec)).
In the mode 1, the interval of the effective symbol in the symbol other than the null symbol is 2048 T (=1 msec) as described above. In the mode 2, the interval of the effective symbol in the symbol other than the null symbol is 512 T (=2048 T/4)(=250 .mu.sec (=1 msec/4)). In the mode 3, the interval of the effective symbol in the symbol other than the null symbol is 256 T (=2048 T/8)(=125 sec (=1 msec/8)). In the mode 4, the interval of the effective symbol in the symbol other than the null symbol is 1024 T (=2048 T/2)(=500 .mu.sec (=1 msec/2)).
Further, in the mode 1, the time of the guard interval in the symbol other than the null symbol is 504 T (=246 .mu.sec). In the mode 2, the time of the guard interval in the symbol other than the null symbol is 126 T (=504 T/4) (=61.5 .mu.sec (=246 .mu.sec/4)). In the mode 3, the time of the guard interval in the symbol other than the null symbol is 63 T (=504 T/8) (=30.75 .mu.sec (=246 .mu.sec/8)). In the mode 4, the time of the guard interval in the symbol other than the null symbol is 252 T (=504 T/2) (=123 .mu.sec (=246 .mu.sec/2)).
In the above apparatus for demodulating the OFDM modulated signal, when the synchronization signal is generated, the original signal which is the OFDM modulated signal is delayed by a time corresponding to the interval of the effective symbol of the symbol. Further, the signals of the periods, corresponding to the guard interval, of the original signal and the delayed signal, are stored in the memory. Then, both of the signals stored in the memory are multiplied with each other and then the product of the multiplication is integrated by the low-pass filter, thereby the correlation being detected. The correlation signal having the rectangular waveform and obtained when the correlation is detected is subjected to interval integration, thereby the signal having the triangular waveform being obtained. The synchronization signal is generated by discriminating a peak of the signal having the triangular waveform. Therefore, if the TFPR symbol (second symbol) of each frame is analyzed, then it is possible to shorten the time required for obtaining synchronization as compared with a system of establishing a frequency and time synchronization.
However, as the time of the guard interval is longer, then the number of multiplications for detecting correlation is increased. This increase requires a lot of time and a larger consumed power both for generation of the synchronization signal. Moreover, this increase requires a memory of a large capacity.